1. Field of the Invention
The present invention relates generally to a projection system, and more particularly to a single-panel projection system with a compact structure and high light efficiency.
2. Description of the Prior Art
In a conventional projection system, an imaging device, such as a liquid crystal display (LCD), a digital micromirror device (DMD), or a liquid crystal on silicon (LCoS), controls the on/off operation of light emitted from a light source on a pixel-by-pixel basis and forms a picture, and a magnifying projection optical system enlarges the picture to be displayed on a large screen.
The projection systems are classified into three-panel projection systems and single-panel projection systems according to the number of imaging devices that are used. The three-panel projection system provides better light efficiency than the single-panel projection system, but is more complicated and expensive, and requires more time in assembly than the single-panel projection system. The single-panel projection system includes a smaller optical system than the three-panel projection system, but provides light efficiency of only one third of the three-panel projection system. More specially, in a single-panel projection system, white light radiated from a white light source is separated into three color beams, which are respectively red (R), green (G) and blue (B) color beams, using color separators, and the three color beams are sequentially sent to one imaging device. The imaging device operates according to the sequence of the color beams to create images. Therefore, synthetically considering the factors of the cost, the assembly process, the light efficiency, and the like, a projection system which has not only high light efficiency but also simple structure is urgently needed in the art.
A color scrolling method, which enhances the light efficiency for the single-panel projection system, has been recently developed. With the color scrolling method, R, G and B beams are simultaneously sent to different locations on an imaging device. Since an image cannot be produced until all of the R, G and B beams reach each pixel of the imaging device, the R, G and B color beams are moved at a constant speed by a color scrolling means.
A conventional single-panel scrolling projection system is disclosed in U.S. Patent Application Publication No. 2002/0191154 A1. As disclosed, white light emitted from a light source passes through first and second lens arrays, a polarization conversion system (PCS), and a condenser lens, and is separated into a red (R), a green (G) and a blue (B) color beams by first through fourth dichroic filters. Each color beam R, G and B is reflected by or continuously transmits through the dichroic filters. For example, the blue color beam is reflected by the first dichroic filter and incident into a first prism. The green color beam is transmitted through the first dichroic filter, reflected by the second dichroic filter, and incident into a second prism. The red color beam is transmitted through the first and second dichroic filters, and incident into a third prism. As the first, second, and third prisms are rotated at constant speed, the color beams R, G and B are scrolled while passing through the first, second, and third prisms. The green and blue beams G and B are then transmitted and reflected by the third dichroic filter 112, and combined with each other. The red, green and blue beams R, G and B are then combined by the fourth dichroic filter. The combined beam is transmitted by a polarization beam splitter (PBS) and forms a picture using an imaging device. The picture is finally projected onto a screen by a projection lens.
Since the conventional projection system uses different light paths for different color beams R, G and B, a light path correction lens must be included for each color beam R, G and B. Therefore, the conventional projection system is bulky, and the manufacture and assembly of the conventional system are complicated, thus decreasing the yield. Furthermore, the first, second, and third scrolling prisms are rotated by three motors respectively, which generates much noise during operation. Additionally, the conventional projection system uses many optical components including at least three prisms and six dichroic filters. This makes it difficult to reduce the manufacture cost.
Hence, an improved projection system is required to overcome the above-mentioned disadvantages of the prior art.